Recently, mobile electronics, such as cellular phone, notebook computer, digital camera, and digital home appliances, such as digital television, DVD (Digital Video Disc) have been increasingly developed in performance and multi-function, requiring higher signal processing speed and signal transmission rate.
Since high-speed signals cannot be transmitted merely with simple wiring design, evaluation of SI is important. Further, superposition of high frequency noise onto power and ground lines may adversely affect operation of electronic parts, such as LSI, hence, evaluation of PI is also important. Furthermore, in a case of a designed ground is not very strong and no path for return current is ensured, common mode current flows through both of signal lines and the ground to generate unwanted radiation (EMI), resulting in electromagnetic interference in television and radio receiver or malfunction of other electronics. These SI, PI and EMI share common issues of a) taking a measure to speed up signals, and b) taking a measure under constraint on design of ground.
As to SI, for example, influence of parasitic impedance accompanied with wires and influence of signal reflection which may occur at input to electronic parts or at joint of wiring, such as via hole, are not regarded as a problem at a lower transmission speed, but will be of importance as speeding up of signals. To cope with this issue, envisaged is design of wiring in consideration of impedance by providing a stronger ground with a larger area. In practice, however, it is very difficult to provide such a stronger ground with a larger area under constraint on downsizing of hardware, high-density mounting, cost-cutting (e.g., the reduced number of layers in multi-layer board), and the like.
As to PI, there is a problematic phenomenon of so-called ground bounce, i.e., electric potential of ground being varying alternately. One factor of this phenomenon is that power and ground lines have a finite impedance (e.g., inductive impedance) and variation of voltage takes place when an AC signal flows. The variation of voltage of ground can be expressed by L·(dI/dt) using the finite inductive impedance L and a current I. This variation is increased with a higher speed and a higher frequency. A weaker ground has a larger impedance, thereby fomenting such a PI issue.
As to EMI, as described above, a main factor thereof is that no path for return current is ensured, which is closely related to strength of ground and frequency.
Measurement and evaluation of the above-mentioned SI, PI and EMI require a technique for measuring and evaluating transmission lines and, power and ground lines having a weak ground at a higher speed and a higher frequency.
Patent document 1 mentioned below relates to a proximity electromagnetic measurement technique, in which an electromagnetic probe is positioned close to electronic parts or transmission lines to measure a quantity of EMI radiated from electric circuits including transmission lines and, power and ground lines having a weak ground, In particular, arrangement of the probe close to power and ground lines enables indirect evaluation of PI.
Next, another technique for directly measuring impedance of transmission lines will be described below.
FIG. 17 is a block diagram showing an example of a conventional measurement system for electric circuit parameters. Here, used for DUT (Device Under Test) is a 2-terminal circuit 70 having a pair of input terminals 74a and 74b, and a pair of output terminals 75a and 75b. 
FIG. 18A is a perspective view showing an example of the 2-terminal circuit 70. FIG. 18B is a cross sectional view along a longitudinal direction of a signal strip line 73. The 2-terminal circuit 70 includes an electrically insulating board 71, a ground pattern 72 located on a bottom face of the board 71, and the signal strip line 73 located on a top face of the board 71. The input terminal 74a is connected to one end of the signal strip line 73, and the input terminal 74b is connected to one end of the ground pattern 72. The output terminal 75a is connected to the other end of the signal strip line 73, and the output terminal 75b is connected to the other end of the ground pattern 72.
As shown in FIG. 17, the input terminals 74a and 74b of the 2-terminal circuit 70 are connected through a signal cable 81 to a network analyzer 80. The output terminals 75a and 75b are connected through another signal cable 82 to the network analyzer 80. The input terminal 74b and the output terminal 75b are grounded to keep a ground voltage.
The vector network analyzer (VNA) 80 can supply the DUT with a reference signal to measure both amplitude and phase of the signal passing through the DUT and amplitude and phase of the signal reflected from the DUT, finally, line characteristics of the DUT, such as S-parameters. Here, the reference signal is supplied through the signal cable 81 into the input terminals 74a and 74b, while measuring the passing signal outputted from the output terminals 75a and 75b of the 2-terminal circuit 70 using the interposing signal cable 82. At the same time, the reflecting signal returned from the input terminals 74a and 74b of the 2-terminal circuit 70 is measured using the interposing signal cable 81, resulting in S-parameters having a 2×2 matrix for the 2-terminal circuit 70.
FIG. 19 is a block diagram showing another example of a conventional measurement system for electric circuit parameters. Here, used for DUT is a 4-terminal circuit 70a having a pair of input terminals 74a and 74c, a pair of input terminals 74b and 74d, a pair of output terminals 75a and 75c and a pair of output terminals 75b and 75d, in which not only the signal strip line 73 of the 2-terminal circuit 70 shown in FIG. 18 but also the ground pattern 72 thereof is regarded as a kind of signal line.
FIG. 20A is a perspective view showing an example of the 4-terminal circuit 70a. FIG. 20B is a cross sectional view along a longitudinal direction of the signal strip line 73. The 4-terminal circuit 70a is configured by arranging two additional dummy metal plates 76 and 77 in parallel above and below the board 71 of the 2-terminal circuit 70 shown in FIG. 18.
The input terminal 74a is connected to one end of the signal strip line 73, and the input terminal 74b is connected to one end of the ground pattern 72. The input terminals 74c and 74d are connected to one ends of the dummy metal plates 76 and 77, respectively. The output terminal 75a is connected to the other end of the signal strip line 73, and the output terminal 75b is connected to the other end of the ground pattern 72. The output terminals 75c and 75d are connected to the other ends of the dummy metal plates 76 and 77, respectively.
As shown in FIG. 19, the input terminals 74a and 74c of the 4-terminal circuit 70a are connected through a signal cable 83 to a network analyzer 80. The input terminals 74b and 74d are connected through a signal cable 84 to the network analyzer 80. The output terminals 75a and 75c are connected through a signal cable 86 to the network analyzer 80. The output terminals 75b and 75d are connected through a signal cable 85 to the network analyzer 80. Accordingly, when a coaxial cable is used for each of the signal cables 83 to 86, the input terminal 74b and the output terminal 75b of the ground pattern 72 are connected via the cable core. The input terminals 74c and 74d and the output terminals 75c and 75d are grounded to keep a ground voltage.
Thus, in case of the DUT being the 4-terminal circuit 70a, S-parameters having a 4×4 matrix can be measured.
FIG. 21 a circuit diagram showing an example of a converter for converting a 4-terminal circuit into a 2-terminal circuit. An input balun 87 is connected to the input terminals 74a and 74b of the 4-terminal circuit 70a, and an output balun 88 is connected to the output terminals 75a and 75b, and the input terminals 74c and 74d and the output terminals 75c and 75d are grounded. Using this arrangement of the baluns 87 and 88 on input and output sides, four unbalanced signals can be converted into two balanced signals.
When this converter is configured on, e.g., a circuit simulator, the resultant S-parameters having a 4×4 matrix for the 4-terminal circuit 70a can be converted into other S-parameters having a 2×2 matrix for the input and output terminals of the 2-terminal circuit having the baluns 87 and 88. In actual measurement, on the other hand, a network analyzer or the like can measure S-parameters for the 2-terminal circuit into which the 4-terminal circuit is converted by this converter.
[PATENT DOCUMENT 1] JP-3394202, B
In such electromagnetic measurement with a probe as Patent Document 1, a spatially distributed electric field or magnetic field is observed. A measured value varies with dependence on a distance between the probe and a measuring object, hence, it is difficult to measure a absolute value. Further, the prove having a larger dimensions has higher sensitivity but lower planar resolution. Accordingly, in a case of fine patterns required for downsizing of hardware, it is difficult to identify a radiating portion.
Meanwhile, measurement of 2-terminal circuit using network analyzer, as shown in FIG. 17, is a relatively common technique. We could find out such problems as follows.
FIG. 22 is an illustrative view showing a current path in the measurement system of FIG. 17. When the ground pattern 72 of the 2-terminal circuit 70 is very strong, a current passing through the signal strip line 73, as shown by dashed line, will return via the ground pattern 72 to the input side. But when the ground pattern 72 is weak, part of the current passing through the signal strip line 73, as shown by double-dashed line, may leak to the output side, and flow through the ground line of the network analyzer 80, thereby causing an error of signal reflection characteristics in the input side.
The measurement of 2-terminal circuit in FIG. 17 is effective in a DUT having an ideal ground, whereas, in another DUT having a weak ground, S-parameters different from actual values will be obtained.
Measurement of 4-terminal circuit using network analyzer, as shown in FIG. 19, requires additional dummy metal plates 76 and 77 which can act as ground, resulting in complicated tools measurement, and increased cost and labor. Moreover, an additive parasitic impedance between the dummy metal plates 76 and 77 and the ground pattern 72 or the signal strip line 73 may cause an error of measurement.
As described above, the conventional measurement technique cannot achieve higher accuracy in any DUT having a weak ground.
Next, strength of ground will be discussed using an example of micro strip lines commonly used in a high frequency. Generally, in case of designing transmission lines composed of micro strip lines, it is assumed that the ground line is an ideal, i.e., very strong ground. In designing characteristic impedance of 50 ohm, for example, width of return line (opposite to ground) out of transmission lines, or thickness or permittivity of dielectric material will be modified.
In a case where a slit is formed across a current flow direction in the ground of the micro strip lines, the characteristic impedance will deviate from 50 ohm, because the ground dose not act as an ideal ground (electric conductor without voltage variation), in other words, the strong ground becomes weaker. The ideal strong ground can be made of an electric conductive plane having a larger area. But actual electronics have finite dimensions, therefore, the ground thereof is also finite. There is a possibility that some transmission lines are provided with strong grounds but other transmission lines are provided only with weak grounds.
As to weak grounds, in addition to the above-mentioned structure having a slit, width Wg of a ground is not very larger than width Ws of a return line out of transmission lines. For example, in case of Ws>5·Wg, the ground becomes weak.
Thus, when replacing a ground having a certain shape by an ideal ground, if characteristics, such as characteristic impedance or S-parameters, of the circuit is altered, it is determined that the ground is a weak ground.
An object of the present invention is to provide a method and an apparatus for measuring electric circuit parameters, which can measure electric circuit parameters, such as S-parameters, Z-parameters or the like, even of a DUT having a weak ground, in a simple way with high accuracy and low cost.